The present invention relates to a hydrometallurgical process for treating nickeliferrous ores and more particularly, to a method of treating low grade nickeliferrous lateritic ores containing large amounts of silica and magnesia and small amounts of nickel to recover iron, nickel and magnesium containing significantly reduced amounts of silica.
The laterite ores which are processed in accordance with the method of this invention are oxidic complexes containing small amounts of nickel and cobalt while containing large amounts of iron and substantially larger amounts of magnesia and silica. The Riddle Oregon nickel laterite ore deposits are illustrative, and a typical Riddle nickel laterite ore, after dryng, analyzes by weight about 0.7% nickel, 0.04% cobalt, 0.5% chromium, 6.5% iron, 36% magnesia, 48% silica and 6% loss on ignition. The amounts of these components will vary somewhat depending on the source of the ore and any preliminary beneficiation treatment.
The nickel values in nickeliferrous oxide ores, such as laterites, are highly dispersed throughout the ore and are present as complex hydrated silicate minerals of varying chemical compositions. Because the nickel values generally are not present as a separate and distinct phase, the ore cannot be beneficiated to produce a concentrate which is rich in nickel.
Laterite ores containing nickel have been treated pyrometallurgically to recover ferronickel, or pyrometallurgically in conjunction with vapometallurgical techniques to recover nickel as nickel carbonyl. These methods are suitable for ores containing above 2% nickel, but the processes are not entirely satisfactory because large amounts of fuel and energy are consumed in heating the mass of the ore which is predominately gangue.
A number of proposals have been made in the prior art for the extraction of nickel from laterite nickel ores which involved direct acid leaching of the raw ore, sulfating, chloridizing, and, after preliminary reduction, leaching with acid or ammoniacal solutions or extraction with carbon monoxide. Most of these proposals are costly, and do not result in an acceptable high recovery of nickel.
Previous proposals for direct acid leaching of the raw ore to extract desirable metal values such as nickel, iron and magnesium, have involved one or more baking operations at elevated temperatures thereby requiring large amounts of fuel and energy. Moreover, some of these techniques proved unsatisfactory for treating laterite ores containing large amounts of silica since the leachng techniques were not successful in separating the silica from the desired metals.
It also has been suggested to selectively reduce nickel-bearing lateritic ores and then to leach selectively reduced nickel values therefrom. Ammonia and acids have been suggested as leaching agents depending on the nature of the ore. The difficulty with this proposed technique is that the selective reduction must be conducted, to insure that the nickel values are reduced and to insure that the magnesia contents are rendered less soluble. Recovery of nickel values using ammonia leaching is not as good as desirable. Acid leaching results in higher nickel recoveries but large amounts of acid are required which results in greater amounts of dissolved silica and formation of silica gel resulting in handling problems.
One example of the prior art technique utilizing a dilute aqueous solution of sulfuric acid followed by baking at an elevated temperature to form extractable sulfates, and finally leaching to extract the desired metals is found in U.S. Pat. No. 3,899,300. A variation in this technique is described in U.S. Pat. No. 3,244,513, where the ore is sulfated with sulfuric acid and heated at a temperature of between 500.degree. and 725.degree. C prior to leaching. These patents are illustrative of the commonly described baking processes which require high capital investment, significant amounts of energy input and several hours for completion.
U.S. Pat. No. 3,868,440 describes a hydrometallurgical process for treating slag materials, especially copper smelter slag material to extract copper as copper sulfate while leaving most of the silica in the slag as insoluble silicious residue. This procedure involves mixng the ground-up slag with concentrated sulfuric acid to form an acid-slag mixture, adding a unit part by weight of water, and allowing the mixture to react to produce a dry solid material in which the metal values are converted to a water-soluble form which is readily separable from the insoluble silica material. The aging of the acid-slag mixture requires at least several hours and preferably from about 8 to 30 hours. U.S. Pat. No. 3,868,440 also discloses that satisfactory results also can be obtained when the acid is added to wet slag, and that this procedure is desirable in processing of water-ground slag.
There still remains, however, a need for a low-cost, low-energy process for extracting nickel and magnesium from nickeliferrous ores which will provide nickel and magnesium values in quantity and of satisfactory purity.